1. Field of the Invention
The present invention relates to battery chargers and, more particularly, to an automatic shut-off battery charging system which intermittently monitors the equivalent internal resistance of a load battery under charge, and which disables charging when the internal resistance indicates that the battery is charged to capacity.
2. Description of the Background
Conventional battery charging systems having an automatic shut-off feature disable further charging in accordance with a predetermined load voltage, load current, time limit, or rise in battery temperature. However, none of these methods indicate the discharge capacity of the battery, and hence, they do not yield an accurate indication of the remaining charge capacity of the battery under charge. Consequently, existing charging systems frequently overcharge the battery, and thereby waste energy and shorten the useful life of battery.
One of the most accurate methods for determining battery capacity is by measuring the specific gravity of the electrolyte. Unfortunately, measurement of a battery's specific gravity is a complex and troublesome task. A more practical method is by measuring the equivalent internal resistance of the battery, i.e., determining the reduction in voltage supplied by the battery as a function of load. The battery's internal resistance is proportional to the level of charge. Hence, saturation may be determined with accuracy. For a great while, this was considered a complex measurement which required a complex electrometer capable of applying a series of low resistance loads directly across the positive and negative terminals of the battery. Such electrometers resulted in a great current drain to the battery which consumed power and induced sparking. Moreover, the temperature of battery could rise to extreme temperatures.
U.S. Pat. No. 5,126,675 issued to Yang provides a partial solution in the form of a simple and inexpensive battery capacity monitor which indicates the remaining capacity of a battery by indirect measurement of the equivalent internal resistance. The Yang '675 device momentarily applies an RC load across the battery terminals and measures the peak capacitive charge which is indicative of the equivalent internal resistance, which in turn can be correlated to the remaining battery capacity.
Although the Yang '675 device succeeds in giving a simpler yet more accurate indication of the remaining charge capacity, the device is only a monitor. It would be greatly advantageous if the theory of monitoring as set forth in Yang '675 could be incorporated as an automatic shut-off feature in a battery charging system.